As semiconductor layer materials of next generation thin film transistors (TFTs), oxide semiconductors have attracted attention in recent years. Among them, TFTs using an amorphous oxide semiconductor such as In—Ga—Zn—O (hereinafter, referred to as InGaZnO) have attracted attention. Oxide semiconductor TFTs can be formed at a low temperature of 400° C. or lower and thus can be formed during interconnection process of an LSI. In addition, due to a very small leakage current and a high withstand voltage, oxide semiconductor TFTs attract attention as high-performance switches. However, in a heat treatment process such as hydrogen sintering performed in manufacturing a semiconductor device including oxide semiconductor TFTs, heat resistance of the oxide semiconductor TFT was low.